declare
fun {Evaluate E}
   case E of plus(A B) then
      {Evaluate A} + {Evaluate B}
   [] minus(A B) then
      {Evaluate A} - {Evaluate B}
   [] times(A B) then
      {Evaluate A} * {Evaluate B}
   [] divide(A B) then
      {Evaluate A} div {Evaluate B}
   [] int(A) then
      A
   end
end

{Browse {Evaluate plus(minus(int(1) int(2)) times(int(3) divide(int(4) int(5))))}}
% => ~1